FIG. 1 shows a transmission and reception circuit or transceiver based on the prior art.
The transmission and reception circuit contains a transmission signal source which sends a transmission signal to a line signal driver LT. The line driver is of differential design and amplifies the received transmission signal. The output side of the line driver is connected via resistors to a transmission signal line for transmitting a transmission and reception signal. The signal line has a particular line impedance.
The transmission and reception circuit contains a reception filter which filters out the signal received via the signal line. An echo cancellation filter or balancing filter connected between the line driver LT and the resistors R is used to simulate the frequency response which the transmission signal experiences up to the input of the subtractor as a result of ZLINE and the reception filter. The echo cancellation filter filters out the transmission signal amplified by the line driver LT. The transmission signal filtered out by the echo cancellation filter is subtracted from an output signal from the reception filter by a subtractor, so that the transmission signal contained in the output signal from the reception filter is compensated for. The output of the subtractor then provides the wanted reception signal, which is supplied for further signal processing to a reception signal processing circuit within the transceiver.
Transmission and reception circuits or transceivers used in broadband communication systems, particularly in xDSL systems, need to satisfy very great demands in terms of power loss.
German patent No. 100 45 721 describes a differential line driver circuit which requires only a low supply voltage and has low power loss. The differential line driver circuit described therein has two input connections for applying a first and a second input signal. The differential line driver circuit also contains two operational amplifiers. The noninverting input on the first operational amplifier is connected to the first input connection on the line driver circuit, and the signal output of the first operational amplifier is connected via a feedback resistor to the inverting input on the first operational amplifier. The noninverting input on the second operational amplifier is connected to the second input connection on the line driver circuit. The signal output of the second operational amplifier is connected via a further feedback resistor to the inverting input on the second operational amplifier. A setting resistor is used for setting gain, the setting resistor being connected between the inverting inputs on the two operational amplifiers. In addition, a first matching resistor connected between the signal output of the first operational amplifier and an output connection on the line driver circuit is provided. A second matching resistor is provided between the signal output of the second operational amplifier and a second output connection on the line driver circuit.
The differential line driver circuit in DE 100 45 721 has a first and a second positive feedback resistor.
The first positive feedback resistor is connected between the first output connection on the line driver circuit and the inverting input on the second operational amplifier. The second positive feedback resistor is connected between the second output connection on the line driver circuit and the inverting input on the first operational amplifier. In the case of the conventional differential line driver circuit, as described in DE 100 45 721, the output impedance of the line driver circuit is matched to the impedance of the signal line. In this context, the output impedance is determined by the product of an output impedance synthesis factor and the sum of the impedances of the two matching resistors.
In the case of the transceiver shown in FIG. 1, based on the prior art, it is not possible to use a differential line driver circuit, as described in DE 100 45 721, because the echo cancellation filter when using a line driver LT with impedance synthesis can no longer be connected directly to the reception-signal-free outputs of the line driver. A line driver LT with synthesized output impedance physically has no reception-signal-free outputs. It is likewise not possible to connect the echo cancellation filter to the signal inputs on the line driver LT, because the nonlinearities of the line driver LT are no longer compensated for on the subtractor. For this purpose, the line driver needs to be of more linear design around the echo suppression of the analog echo cancellation filter or balance filter, i.e. in an order of magnitude of between 20 and 30 decibels. This in turn disadvantageously results in larger quiescent currents in the line driver LT and thus in higher power loss. It is likewise not possible to connect the echo cancellation filter between the synthesized portion (RSYN) and the physical portion (R) of the terminating impedance. In this case, a large part of the reception signal is suppressed by the subtractor.
On account of the aforementioned drawbacks, there has therefore previously been no use of synthesized line drivers LT, i.e. of line drivers which have a synthesized output impedance, in broadband communication systems containing echo cancellation filters.